This is an unfortunate situation for everyone involved. The end of a
marriage is an emotional and trying time for any family, and mine is
no exception.
As a firefighter and first responder, I have witnessed the reality of
domestic violence and provided care for many victims.
As a state legislator, I remain strongly in support of erring on the side
of protecting potential victims with our laws and their enforcement -
even when, in situations like mine, it can result in unwarranted
charges. I am confident that once the facts come to light I will be
exonerated.
I believe in our justice system and will continue to cooperate fully as
this situation is resolved.

The news was apparently first reported at horsesass.org. Will Kelley-Kamp, one of the writers there, recently took a break from writing at the liberal blog to become Simpson's campaign manager. Horsesass founder David Goldstein doesn't name any sources, but says he's been told:

No actual physical violence was alleged or observed, but state law apparently provides police officers little discretion under these circumstances, even when all parties involved do not want the arrest to happen.

Simpson represents the 47th, a swing district. His close win in 2000 helped put Democrats in a tie with Republicans in the state House. He is looked at by some as something of a Democratic hero. A liberal blogger Noemie Maxwell wrote at DailyKos:

Simpson's presence in the House changed Washington's political landscape in a big way. His win shifted the Democrat-Republican balance to a 49-49 tie, making for a dramatic year. The budget negotiations, for example, which went overtime in a year where legislators had to struggle with the shortfalls created by Eyman's disastrous I-695, would have resulted in a very different outcome if Republicans had had unrestrained control.

Simpson's win also changed my views of politics. I was in a distraught state over the war and the Bush administration and feeling isolated, angry, and intimidated. The mood around me was that, if you were against the war, you were unpatriotic. I was hostile to the Democratic Party, which had allowed our country to be sold out to the military industrial complex that Eisenhower warned us about -- and not a likely partisan volunteer. That began to change after I happened one day upon the text of House Memorandum 4008, Opposing a Preemptive Attack on Iraq. The prime sponsor was my own legislator, Geoff Simpson, and it occurred to me for the first time that a politician could be sincere. The text of this memorandum reads like a prophecy. Soon after, I discovered the local, grassroots Democratic organizations and began to learn from the people and experiences there that involvement in politics is not collusion with evil, but a way to bring citizen input into the process and make it cleaner.

UPDATE: House Clerk Barbara Baker put out this statement last night:

The House of Representatives takes criminal charges against any of the members very seriously - especially those of domestic violence. However, as an attorney, I am advising Speaker Chopp to thoroughly understand the details of this incident before requesting any official action from the institution.

I will be talking later this evening with the Speaker and other members of leadership about this unfortunate situation. Our next steps will be based on what we learn in the days ahead.